The principal goal of this research project is the establishment of the most accurate and clinically useful method by which platelet matching can be performed for use in alloimmunized recipients. This will entail in vitro and in vivo comparisons between the four major techniques now available for platelet matching: HL-A lymphocytotoxicity, 3H-serotonin platelet release, platelet aggregation, and platelet complement fixation. These studies will define the sensitivities of the assays for detecting platelet alloantibodies. The correlation between platelet matching using lymphocytotoxicity and the direct platelet assays with both monospecific HL-A antisera and platelet alloantibodies will be evaluated. The study will assess the degree of HL-A matching required for clinically useful platelet transfusions in patients previously sensitized with random platelet transfusions. An objective evaluation will be made of the premise that there is no hemostatic effect with platelet transfusions unless there is a post-transfusion platelet count rise. The nature of the platelet HL-A antigen will be explored along the lines established for HL-A antigens of the lymphocyte. A selected platelet HL-A antigen will be solubilized and attempts at purification and chemical identification will be undertaken with the final goal being the production of a monospecific xenogenic antibody to this antigen.